1. Field of the Invention
The present invention relates to structures for temperature sensors and infrared detectors.
2. Description of the Related Art
Infrared detectors, so-called IR-detectors, are divided generally into two main groups, namely photon detectors and thermal detectors.
Photon detectors are based on the immediate excitation of charge carriers by IR-absorption, wherein the change level of the charge carriers is detected electronically. Photon detectors are fast and sensitive, but have the drawback of needing to be cooled to cryogenic temperatures.
Thermal detectors are based on a two-stage process wherein IR absorption occurs in a detector structure which is heated thereby. The temperature change is detected by means of an integrated temperature sensor. Examples of thermal detectors are resistive bolometers, thermoelements, pn-diodes, pyroelectric sensors, etc. Thermal detectors have the advantage of being able to operate at room temperature, but have the disadvantage of being less sensitive and slower than photon detectors. Sensitivity and speed are not of paramount importance in the case of detector matrices, and consequently non-cooled thermal detectors are of the greatest interest with regard to detector matrices.
Irrespective of whether IR-radiation is measured with thermal detectors that are based on the increase in temperature of a detector structure due to IR-radiation, or whether the detector structure is heated as a result of being in heat conducting contact with a material whose temperature is to be measured, the problems encountered with known temperature sensors are the same.
The problem with known temperature sensors, or thermal detectors of the aforesaid kind, is that either the output signal is weak or the noise level is high, and consequently a common drawback is that the signal-noise ratio is low.
A sufficiently high signal-noise ratio is necessary in order to obtain a high degree of sensitivity. When measuring rapid temperature changes, the possibilities of filtering out or integrating away the noise are limited.
In the case of resistive bolometers, the material or structure must therefore have a sufficiently high temperature coefficient and a low noise level.
Known bolometer materials are either metals or semiconductor materials. Although the latter materials have the advantage of a high temperature coefficient they, unfortunately, also have a high noise level. The noise may be of a fundamental nature, such as Johnson noise or generation-recombination noise, meaning that the noise is difficult or impossible to reduce. Alternatively, the noise may be 1/f noise, where f is the frequency, deriving from poor electrical contacts, impurities, contaminants, and so on. When a sensitive detector is required, it is often necessary to use a semiconductor material as the bolometer layer. Metals can be used only when the bias voltage can be high, which often results in high power generation. This is normally unacceptable in detector matrices that include a large number of detector elements.
Amorphous or polycrystalline material deposited from a gas phase is normally used as semiconductor bolometer material. This is because the layer is normally applied on thin silicon nitride films. The amorphous or polycrystalline nature of the layers gives rise to 1/f noise in both the layers themselves and in the electric contacts with the surroundings. This noise is generated by the multitude of grain boundaries in a polycrystalline structure, among other things.
The present invention solves the problem of a low signal-noise ratio, by using a special thermistor material. Furthermore, according to the invention, a semiconductor material can be designed to enable its temperature coefficient to be selected.